Refactor fetching board lists from Vuex actions to Apollo queries

What does this MR do and why?

• Introduces :apollo_boards feature flag
• Fetches board lists using Apollo query instead of VueX

How to set up and validate locally

1. Turn on the feature flag. In your terminal, run:

bin/rails console
Feature.enable(:apollo_boards)

2. Visit a board (issue project, issue group and epic board) and verify that the board is loading.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #378884 (closed)